ios: set Xcode C++ language standard for Apple framework build

OpenCV requires C++17, and bundled 3rdparty libraries such as KleidiCV
explicitly request CXX_STANDARD 17. With the CMake Xcode generator,
however, neither the global CMAKE_CXX_STANDARD nor per-target
CXX_STANDARD is emitted as CLANG_CXX_LANGUAGE_STANDARD in the generated
project, so those sources are compiled at the toolchain default
(pre-C++17) and fail, e.g.:

  kleidicv/include/kleidicv/traits.h: error: no template named
  'is_base_of_v' in namespace 'std'; did you mean 'is_base_of'?

Set CLANG_CXX_LANGUAGE_STANDARD=gnu++17 at the project level via
CMAKE_XCODE_ATTRIBUTE_* in the shared getCMakeArgs(), so every target in
the generated Xcode project (including 3rdparty subprojects) builds with
C++17.

platforms/ios/build_framework.py

@@ -218,6 +218,12 @@ class Builder:
             "-DOPENCV_INCLUDE_INSTALL_PATH=include",
             "-DOPENCV_3P_LIB_INSTALL_PATH=lib/3rdparty",
             "-DFRAMEWORK_NAME=%s" % self.framework_name,
+            # With the Xcode generator CMAKE_CXX_STANDARD / per-target
+            # CXX_STANDARD is not translated into CLANG_CXX_LANGUAGE_STANDARD,
+            # so bundled 3rdparty libraries that require C++17 (e.g. KleidiCV)
+            # are compiled at the toolchain default and fail to build. Set the
+            # standard explicitly at the project level.
+            "-DCMAKE_XCODE_ATTRIBUTE_CLANG_CXX_LANGUAGE_STANDARD=gnu++17",
         ]
         if self.dynamic:
             args += [